Operational amplifiers (op amp) and comparators are widely used in integrated circuits because their high gains, two inputs, high input impendence, and many other special characteristics. FIG. 1 shows the most popular symbol for an op amp 100. There are two input terminals: inverting and non-inverting, named by their polarity comparison with the output (out) signal. The op amp compares these two inputs with each other to generate the desired output.
When an op amp or comparator performs comparisons, the references can be specified, or may be hidden in an incoming signal. In FIG. 2, fully differential inputs 200, 202 have a hidden reference, and the reference can be found by averaging both input signals. On the other hand, for single-ended signal, one of the inputs may provide fixed value reference for the other to do the comparison.
FIG. 3 shows different ways to create one reference signal for an op amp or comparator. For the fixed reference-based circuits 300, 310 shown in FIG. 3(a) and FIG. 3(b). the reference is independent of input signal strength, so these fixed reference structures are not desirable for applications having a large input dynamic range. In the circuit 320 of FIG. 3(c), the op amp or comparator 100 is configured in a closed-loop configuration. Because of the feedback loop 322, the reference is dynamically adjusted by the op amp or comparator output. However, this configuration is not suitable for open loop applications.
FIG. 4 illustrates a circuit 400 in which the input reference to an op amp 404 is set by a low pass filter 402 created by the combination of the resistor R and capacitor C. The low pass filter filters out signal AC components. The filtered-out AC components are higher frequency than the low pass filter (e.g., −3 dB) corner frequency. The DC level, and lower than low pass filter corner frequency components are retained as a reference voltage Vref for comparing with the original signal Vin. This configuration accommodates a large input dynamic range. However, the low pass filter 402 has a fixed frequency bandwidth, e.g., −3 dB. When the input signal Vin has widely varying data rates, the higher data rate input results in a longer time for the reference signal to settle. The case shown in FIG. 5(a) shows a quick settling because the input frequency is slower than the FIG. 5(b) case. Quicker settling provides an average reference signal level in real time faster, so a comparator or op amp 404 can compare more real reference signal as compared to that of a comparator or op amp using a reference signal having a slow settling time. Furthermore, if the low pass filter has a very low (e.g., −3 dB) corner frequency, the size of the resistor R and capacitor C are very large, which might not be feasible for implementation. However, because of the simplicity and prevalence of resistors and capacitors in filtering, and because most applications have heretofore used a known signal frequency, those skilled in the art have not been motivated to use other filtering schemes.
It would therefore be desirable to achieve faster settling of the reference signal at higher data rates.